In the United States, approximately 40% of energy consumption is used to heat and cool buildings. In buildings, the majority of energy loss takes place through the building envelope. The building envelope consists of doors/windows, exterior wall systems and roofing systems. In addition buildings should not only be energy efficient but also should be able to withstand natural disasters, such as floods, hurricanes, tornadoes, earthquakes, and the like. Therefore, building envelopes needs to be both resilient and highly energy efficient.
Framed walls use metal or wood studs to build a frame that can be either loadbearing or infill. Multistory buildings can be made from cast-in-place concrete with the exterior perimeter walls being in-filled frame construction. Exterior sheathing is attached to the outside of the frame. On the inside, drywall is typically used for the inside finish surface. This framing system creates a cavity between the exterior sheathing and the drywall. This cavity is then filled with batt insulation to improve energy efficiency. It is assumed that the R-value of the batt insulation determines the energy efficiency of the wall system. However, there are several drawback of this system. Framing members create thermal bridging. Batt insulation may not completely fill the cavity wall and over time it can sag, leaving no insulation in some places. Moisture condensation inside the cavity wall is common which dampens and compresses the batt insulation. When this occurs, the damp batt insulation loses most, if not all, insulating properties. HVAC systems create pressure differentials between the interior and the exterior of the building. These pressure differences cause air to move through the exterior wall system. Simply stated, cavity wall framed systems have poor energy efficiency, among many other problems. In addition, framing construction has a very poor record sustaining storm and flood damage. More and more jurisdictions require use of resilient home construction systems. In fact FEMA has an entirely new certification for resilient homes and means to prevent damage arising from natural disasters.
Exterior walls can also be made of concrete, either pre-cast or cast-in-place. Concrete is a composite material comprising a mineral-based hydraulic binder which acts to adhere mineral particulates together in a solid mass; those particulates may consist of coarse aggregate (rock or gravel), and/or fine aggregate (natural sand or crushed fines). While concrete provides a long lifespan and increased protection from damage, concrete is as cold or as hot as the ambient temperature. Concrete has high thermal mass, which makes it rather expensive to heat or cool in extreme temperatures. In an attempt to alleviate this problem, the inside of a concrete building may be insulated. However, such insulation does little to improve energy efficiency as it is generally on the wrong side of the wall; i.e., the interior wall surface. Concrete walls have the advantage that they are barrier systems; i.e., no air can flow through from inside to the outside, but still have poor energy efficiency. While concrete-type building construction does very well in storms and floods, it does not do as well in seismic areas due to its massive weight and minimal flexibility.
Precast or structural concrete wall panels are known in the art. The use of precast concrete wall panels has gained in popularity because they can be manufactured at a remote location, transported to a job site and attached into place, usually be welding steel embeds to a building's steel structural frame. Precast structural panels can also be formed both onsite and offsite and used to support a loadbearing structure of one to four stories tall. Precast concrete panels can be reinforced using standard deformed steel reinforcement (rebar) or stressed cables, such as pre-stressed or post tension cables. Generally these concrete panels are of a uniform thickness, the thickness of which is determined by the anticipated stresses to be placed upon the concrete panels or structure.
Prior art precast concrete wall panels also have a large thermal mass when exposed to ambient temperatures. They retain the heat in the summer or the cold in the winter very well. Therefore, buildings built with precast concrete panels generally have relatively poor energy efficiency. Such buildings usually require a relatively large amount of energy to keep them warm in the winter and cool in the summer. Since most precast concrete panels are not insulated, they must be insulated on the inside through the use of interior framing systems. This method however does not create a highly energy efficient building envelope. And, since batt insulation of significant thickness is required the interior frame system takes valuable floor space and creates a cavity wall.
More recently, new methods of insulating precast concrete panels have been employed. There are a number of insulated concrete panel systems currently employed. All of them are a “sandwich” type panel. Such panels require placing a layer of foam between two relatively thick layers of concrete. Some panels are non-composite while others are composite types. Regardless of which type is used, all concrete insulated sandwich panels are made of uniform concrete thickness on each respective side of the foam panel.
One method involves placing a layer of insulation between a structural concrete layer and an architectural or non-structural concrete layer during the casting of the panel and then erecting this entire non-composite construction as an exterior panel. While this method improves the insulating properties of a wall and therefore the energy efficiency of a building, it has several drawbacks. Instead of having one layer of concrete, the “sandwich” creates two; one that is structural with the larger thermal mass that faces the inside of the building and is insulated from the elements. The second layer of concrete is slightly thinner and placed on the exterior of the building; i.e., on the side of the panel opposite the insulated structural layer. Although the second layer is thinner than the first layer, it usually includes steel reinforcing bars (“rebar”). Rebar has to have a minimum embedment of ½ inches from the exterior face of concrete and is usually placed in the center of the concrete. Therefore, the thinnest exterior concrete is still approximately 3 to 4 inches thick of uniform thickness of each respective layer. The second layer is therefore still relatively thick and heavy. The weight of the second layer added to the weight of the first layer makes the entire panel relatively heavy. The American Concrete Institute and industry practice requires that no shear forces be exerted by the first and second layers of the “sandwich” on the insulating layer. Therefore, a bond breaking layer is applied to the insulating layer so that neither the first nor the second layer will adhere thereto. Since there is no bond between the two layers of concrete and the foam, the ties used to connect the two concrete layers have to be engineered to resist the shear pressure from the weight of the second layer of concrete. Generally this is a costly system.
Other methods of sandwich panel construction involve a layer of foam between two wythes (layers) of concrete in a composite type assembly. The inner and outer wythes can be the same thickness or the inner wythe can be thicker while the outer wythe can be thinner. Some use composite plastic ties to hold the two wythes together while others use carbon fiber mesh. Some sandwich panels use pre-stressed cables to achieve the required strength while others use internal trusses. However these panels are heavier and therefore more expensive to manufacture. Since the exterior wythes are made from conventional concrete, they are still considerably thick due to minimum steel embedment code requirements. The thinner the concrete wythes, the more brittle they become which requires use of pre-stressed cable reinforcement or expensive carbon fiber reinforcements. To place the steel embedments, attachments and reinforcement, the thinnest practical concrete thickness is limited to approximately 2 to 3 inches of uniform thickness of each respective wythe.
Concrete structures and panels are used to provide the load bearing capacity and to carry the loads or stresses of the structure. Vertical panels or walls are used to carry the roof loads and the load of intermediate floors. Horizontal slabs are used to carry the live loads, such as furniture and occupants of a structure. To achieve these properties the concrete has to be reinforced with steel. Concrete structures and panels have to be designed to safely withstand various type of loads or stresses, such a dead loads, live loads, wind loads, and seismic loads within an appropriate amount of deflection. However some of these loads are not equally distributed along a structure or panel. For instance, on each side of an opening there are greater stresses than in the middle of a long span wall. Additionally, building corners have greater stresses than the middle of a building side wall. Certain elevated slab or roof elements are connected to the walls at certain locations thereby distributing a larger load in that specific area than another. At these locations additional steel is used to reinforce the concrete. However the overall thickness of a concrete panel or slab is generally determined by the maximum concrete thickness required in the areas of maximum stress. Therefore, a concrete panel's or slab's thickness is the same in areas of maximum stress as in the areas of minimal stress and consequently is of a uniform thickness. Also, since steel reinforcement has to be continuous, generally the type, size and amount of steel from the areas of maximum stress are carried over into the areas of minimal stress. This creates an unnecessary amount of concrete and steel used in the areas of minimal stress that is not needed. While this is a known factor, the limitation of construction practices makes it impractical and expensive to form concrete panels with various concrete thicknesses and varying steel reinforcement to accommodate the various stresses within a concrete panel or slab. In addition, the aesthetic appearance of a concrete panel with various structural reinforcing elements cast within may not be desirable.
Almost all precast, tilt-up and concrete slabs are made of concrete of uniform thickness throughout. The insulated concrete sandwiched panels mentioned above also have concrete slabs of uniform thickness throughout.
Precast concrete panels are also used to construct highway noise barriers. Concrete noise barriers are used to deflect noise away from the protected areas. Concrete panels cannot absorb noise; they only deflect it. It is know that foam panels can absorb sound. Some states have used foam panels for sound barriers. However the structural limitations of the foam panels make them prone to other shortcomings. Some states, such as Georgia, have discontinued the use of foam panels. Also, while concrete noise barriers may be longer lasting, they are heavy and have very limited architectural features.
Generally roof structures are built using a system of steel beams, steel roof joists and corrugated metal roof deck. To provide insulation to the roof, insulation board is attached to the metal deck using fasteners generally spaced 24 to 36 inches on center. A roof membrane is then attached to the foam board using an adhesive. In certain cases, the roof membrane is attached using fasteners. While such roof systems are very popular, they are highly susceptible to storm or wind damage.
To create a roof system that can withstand hurricane force winds, concrete is typically poured on top of the corrugated metal deck. Then, insulation is attached to the top of the concrete. In some cases lightweight concrete is poured on top of the metal deck. Since lightweight concrete is a better insulator than regular concrete, some projects will attach a roof membrane directly to the top of the concrete without any insulation. While this provides greater wind resistance, it is not a very energy efficient roof system.
The biggest drawback of any roofing system that uses poured concrete on a metal roof deck is that the metal deck acts like a pan and it collects moisture. Since concrete needs to be moisture cured in order to achieve its maximum strength, additional water may be sprayed onto the concrete. Furthermore, lightweight concrete has significant amounts of air pockets and types of aggregate that retain water. Due to weather and construction schedules, roof membranes are sometimes applied while there is still significant moisture in the concrete. This moisture retained by the concrete is therefore trapped between the metal deck on the bottom of the concrete and the roof membrane on the top. Due to weather cycles, this trapped moisture has nowhere to go but up thereby causing failure of the roof membrane with resulting potential severe damage to the interior of the building.
Due to the specific design limitations, precast insulated sandwich concrete panels are seldom if ever used for a roof deck.
Therefore, it would be desirable to provide a system for relatively easily and efficiently insulating precast concrete panels or other structures to achieve the highest energy efficiency possible. It would also be desirable to provide a precast concrete panel system that provides a concrete form for casting structural reinforcing elements within the precast cementitious-based or cementitious panel or slab. It would also be desirable to provide a concrete panel that uses reduced amounts of concrete and reinforcing steel compared to conventional concrete panels or slabs. It would also be desirable to provide a composite precast insulated concrete panel that is lighter and stronger than prior art panels so that it can have improved performance in any type of natural disaster. It would also be desirable to provide an insulated precast concrete roof deck that does not trap moisture in the roof system. It would be desirable that the insulated precast concrete roof panels have greater energy efficiency and wind load resistance.
It would also be desirable to have a highway noise barrier system made of composite precast insulated concrete panels that are both sound absorbing as well as sound reflective. It would be desirable that such highway noise barrier system panels have the option to integrally include a wide range of architectural finishes. It would also be desirable to provide an integrated architectural finished composite precast insulated concrete panel that can incorporate all necessary reinforcing elements required by localized stresses within the panel or slab. It would also be desirable that such panels efficiently integrate a wide variety and types of cladding, finish textures, colors, and patterns, such as concrete, plaster, stucco, stone, brick, tile and the like.